A silver halide grain containing two or more parallel twin planes has a tabular form (hereinafter referred to as a tabular grain). Said tabular grain includes the following features as a photographic characteristic.
1) A ratio of a surface area to a volume (hereinafter referred to as a specific surface area) is large and a lot of a sensitizing dye can be adsorbed on the surface thereof. That results in providing a relatively high color sensitized sensitivity.
2) Since in the case where an emulsion containing the tabular grain is coated and dried, the grains thereof are aligned parallel to a surface of a support, a light scattering by the grains can be reduced thereby improving in a sharpness and a resolution of images. Further, said alignment can allow a thickness of a coated layer to be thinned and the sharpness of the images can be improved.
3) Since the specific surface area is large, a development rate can be accelerated.
4) Since a covering power of the silver halide emulsion is high, an amount of the silver can be saved.
Thus, the tabular grain has many advantages and therefore, it has so far been used for a commercially available light-sensitive material having a high sensitivity.
The emulsion grains having an aspect ratio of 8 or more are disclosed in JP-A-58-113926 (the term "JP-A" as used herein means an unexamined published Japanese patent application), JP-A-58-113927 and JP-A-58-113928. The aspect ratio referred herein means a ratio of a diameter to a thickness of the tabular grain. Further, the diameter of the grain means a diameter of a circle having the same area as a projected area of the grain. The thickness means a distance between the two paralleled subject surfaces which constitute the tabular grain.
However, as can be seen in the examples of the referenced patents described above, the tabular grains prepared by the conventional processes have an inferior monodipersibility due to the following reasons.
(1) The tabular grains show broad distribution in a projected diameter area, and PA1 (2) in addition to the tabular grain, there are mixed a stick-shaped grain, a tetrapod-shaped grain, a singlet twin grain and a grain having a non-parallel twin plane. PA1 (A) enhanced contrast of image (so-called high gamma) in a characteristics curve can not be expected, PA1 (B) in the case where an emulsion in which large grains and small grains are present by mixture is subjected to a chemical sensitization, it is difficult to provide an optimum chemical sensitization to the both since the large grain and the small grain each has a different requirement for the chemical sensitization, and PA1 (C) a multi-layer system, wherein monodispersed large grains are present on an upper layer and monodispersed small grains are present on a lower layer, provides higher sensitivity rather than an emulsion-coated layer wherein the large grains and the small grains are present in a mixture in terms of a utilizing efficiency of a light. However, this system has such a defect that this effect can not sufficiently be utilized.
These features may cause the defects as follows:
Accordingly, various attempts for a monodispersion of the tabular grain have so far been made and several patents have been disclosed. The monodispersed tabular grains disclosed in JP-A-52-153428 has a restriction that an AgI crystal should be used as a nucleus, and a grain formed has a tabular grain in a small ratio. A grain growth condition for the monodispersing the tabular grain is disclosed in JP-A-55-142329 but the grains formed have a low ratio of the tabular grain. The monodispersed twin grains disclosed in JP-A-51-39027 are prepared by a process which comprises forming a nucleus, adding a silver halide solvent for ripening, followed by growing thereof, however, the grains obtained show a low ratio of the tabular grain which has a low aspect ratio as well. The monodispersed twin grains are also included in JP-A-61-112142 for a grain-forming process, in which a spherical grain is used as a seed crystal, however, an aspect ratio of the tabular grain is 2.2 or less and only a tabular grain emulsion having a low tabular grain ratio was obtained. The monodispersed tabular grains described in French Patent 2534036 are formed by a process comprising forming a nucleus, and ripening without using a silver halide solvent to obtain the grains having a fluctuation coefficient (a value which is obtained by multiplying a value obtained by dividing a standard deviation of a circle-corresponding diameter with an average circle-corresponding diameter by 100) of 15% in the circle-corresponding diameter of the tabular grains.
According to a calculation from a grain shown in photo which is attached to the examples of French patent, a projected area of a triangle tabular grain shares 50% or more is demonstrated. According to J. E. Maskasky, J. Imaging Sci., 31, 1987, pp. 15 to 26, said triangle tabular grain is a grain having three twin planes which are parallel to a main surface.
The monodispersed tabular grains containing the hexagonal tabular grains are disclosed in JP-A-63-11928, JP-A-63-151618 and JP-A-2-838. The hexagonal tabular grain is a grain having two parallel twin planes unlike the triangle tabular grain described above. In Example 1 of JP-A-2-838 described above, the monodispersed tabular grains in which the tabular grains having two parallel twin planes share a ratio of 99.7% in the whole projected area and a fluctuation coefficient in a circle-corresponding diameter is 10.1%.
The processes in which the monodispersed tabular grains are obtained by allowing a polyalkylene oxide block copolymer to coexist in forming a nucleus are disclosed in U.S. Pat. Nos. 5,147,771, 5,171,659, 5,147,772, and 5,147,773. Further, a monodispersed tabular grain emulsion having a fluctuation coefficient of 10% or less is disclosed in European Patent 514742A, wherein the polyalkylene oxide block copolymer described above is used in all of the examples.
However, while forming the tabular grains according to this example leads to formation of the monodispersed tabular grains, the hexagonal tabular grains are of a distorted form in which each length of the six sides is different one another at random. In general, it is known that among the arrangements of the respective pixel in an image sensor, an ideal pixel arrangement is a honeycomb structure, and preferably, an equilateral hexagonal form (J. C. Dainty and R. Shaw, Image Science, Academic Press, London, 1974). Accordingly, the formation of the hexagonal tabular grains having a more uniform shape has been desired. While it has been observed that application of the monodispersed tabular grains thus obtained to a silver halide photographic material leads improvement in a graininess and a sensitivity, it includes the problems that the improvement is not sufficient and further that this tabular emulsion does not necessarily have a sufficient preservability (a latent image preservability) lasting to a development of a photographic material after photographing, and further improvement has been desired,